Diffusion of Zn into InP, InGaAsP and InGaAsP/InP single heterostructureshas been studied. The depth of the diffusion front is found to be proportional to the squareroot of the diffusion time. For single heterostructures the junction depth is dependent onthe InGaAsP epilayer thickness x_0, i.e. x_j/t~(1/2)=-x_0/(rt~(1/2))+I, which is very useful inthe fabrication of many electronic and optoelectronic devices where heterostructures areused and Zn diffusion is necessary.